1. Field Of The Invention
This invention relates generally to the magnetic tape art. More particularly, it is directed to a magnetic tape erasing system in a miniaturized cartridge-type tape drive which erases a tape utilizing a series resonant AC erase head drive circuit.
2. Discussion of the Prior Art
The prior art with respect to this invention lies within the miniaturized cartridge-type tape drive art for use with data processors. An example of such a tape drive is described in U.S. Pat. No. 4,647,994 in a patent to Irwin, et al. entitled "Miniaturized Cartridge-Type Drive for Data Processors". Such tape drives typically have a peripherally-enclosing main frame providing an access opening for insertion of a tape cartridge and structurally mounted, pancake-drive motors for tape transport and head-positioning. This construction allows accessing of any of a plurality of adjacent recording tracks on the tape.
The present invention is specifically directed to the erasure of tape media transported on such tape drives. In one U.S. Pat. No. 4,466,027 to Howell et al, entitled "Digital Tape Erasure Conditioning System", a tape erasure conditioning system is disclosed. The Howell patent utilizes alternating current (AC) erasure which provides an erase field that alternated rapidly relative to the movement of the media across the erase head. The erasing circuitry of the Howell patent provided a 3.58 MHz square wave to two erase drive transistors. The inductance of the erase head in parallel with stray capacitance and a resonating capacitor, allows for a resonant circuit to pump the erase head until a self-limiting condition occurs, thereby AC erasing the previously magnetized tape.
There are several limitations of the prior art erase head circuits which are overcome by the present invention. The Howell patent, for example, is difficult to implement for frequencies above about 3.58 MHz. Further, the approach taken by Howell may require the use of a high voltage power supply or transformers. The scheme employed by Howell requires the use of a center tapped winding. This center tapped scheme makes the use of series resonance or AC coupling impossible, as all 3 wires carry DC current. In a center tapped scheme, such as employed by Howell, the only resonance one can employ is parallel resonance in a circuit employing a resonant capacitor in parallel with the coil. Since the erase head includes an inductance component, the use of parallel resonance only makes supplying current into the inductor coil in the erase head more difficult by making the network comprising the erase head and capacitor impedance higher, not lower.
In contrast, the series resonance scheme of the instant invention cancels the erase head inductance with series capacitance. It also features simple and precise voltage control of the AC erase current. The instant invention provides a circuit capable of delivering a significant amount of power to the erase head at a very high frequency of about 7 MHz. The instant invention enables precise control of the AC erase current by employing an easily generated control voltage. Such a control voltage may be generated from a potentiometer or a microprocessor-based system using well known methods.
Currently, there is a need in the cartridge tape field to erase 900 Oe tape which is a replacement for previously used 550 Oe tape. It has been discovered that 900 Oe tape requires about 40% more current to accomplish erasure down to-30 db on a relative db scale. At high frequencies, the erase head drive circuit of the instant invention provides the requisite current and voltage to the erase head in a much easier manner than the center tapped drive circuit of the prior art.